Many techniques used in the fabrication of electronic integrated circuit chips lend themselves readily to micromachining of mechanical devices such as valves. Microfabrication of mechanical devices is discussed by Angell et al. in "Silicon Micromechanical Devices," Scientific American (April 1983), pp. 44-55. Fabrication of a microminiature valve for use in gas chromatography is described. The analysis of gases in a silicon gas chromatograph is based on differences in the solubility of various gases in a liquid which lines the interior wall of a capillary through which the gases flow. Microminiature valves are used as gas flow regulators, as for example in setting the flow of a carrier gas through such a capillary. Some mechanical actuation means must be provided for such valves.
Solenoid actuation of a valve in a gas chromatography assembly is described in Terry et al. U.S. Pat. No. 4,474,889. However, John H. Jerman, a coinventor of the Terry et al. device, noted in a June, 1990 IEEE transaction that such actuation is not attractive because of the difficulties involved in providing sufficient actuation force ("Electrically-Activated, Micromachined Diaphragm Valves" by Jerman, Technical Digest, IEEE Sensor and Actuator Workshop, June 1990, pp. 65-69). Other difficulties associated with solenoid actuated valves are that they are expensive and that a substantial portion of such a valve cannot be batch-fabricated by using known microfabrication technology.
Other means for actuating a micromachined valve are known. O'Connor U.S. Pat. No. 4,581,624 teaches use of electrostatic force to deflect a flexible diaphragm until the diaphragm seals an outlet aperture valve seat. However, providing sufficient force for reliable actuation is a problem. Sittler et al. U.S. Pat. No. 4,869,282 teaches a micromachined valve which is actuated in part by gas pressure differentials at various ports of the valve. Such a valve is necessarily complex, and requires control gases to operate.
The above-identified paper by Jerman teaches use of a bi-metallic diaphragm consisting of a pair of materials, not necessarily metals, which are bonded together. The micromachined bi-metallic diaphragm has a lower surface of silicon and an upper surface of aluminum. As the temperature of the diaphragm is changed, stresses that are generated in the structure cause a deflection which moves the diaphragm and a downwardly-depending center boss relative to an outlet surrounded by a valve seat. In a normally-open embodiment, the center boss is moved toward the valve seat by the heat-induced deflection of the diaphragm. This deflection closes an otherwise open path to the outlet, thereby cutting off a flow of fluid to a system.
The bi-metallic structure taught by Jerman follows the teachings of the prior art. That is, the bimetallic structure is a solid circular diaphragm which is deflected to regulate fluid flow. The improvement of the structure is that the solenoid actuator of Terry et al. is replaced by a deposit of aluminum on a silicon diaphragm layer. Thus, the bi-metallic structure may be batch-processed in its entirety using known microfabrication technology. However, the valve is less than ideal. One problem involves the nonlinear deflection vs. force characteristics of the diaphragm. A microminiature valve may be required to open or close against a pressure of 200 pounds per square inch (psi). A diaphragm displacement of 40 microns may also be required. Such diaphragm displacement varies as the cube root of actuation force for large displacements, and this effect nonlinearity disproportionately increases in significance with an increase in deflection. In the deflected state, the valve is wasteful, since little deflection, and therefore little work, is performed by increases in force after a significant opening has already been achieved. Moreover, the bi-metallic diaphragm raises new considerations, such as thermal isolation of the diaphragm from the frame which supports the diaphragm to avoid excessive heat loss. It is important that the power supplied to a microminiature valve be efficiently utilized, but Jerman does not teach an efficient valve.
It is an object of the present invention to provide a microminiature valve which efficiently produces work throughout the entirety of a large range of displacement.